Boffins help please... Calculating power output when using rollers.

I've just switched to using Kreitler rollers after 6 months of Minoura. My new ones have a slightly smaller drum diameter. I wanted to compare my training now with several months ago. I see on the Kreitler website that they have a guide to power output at certain speeds and that they also state that rider weight will affect this. I wondered if anyone knew of a formula for calculating power output that would use drum diameter and rider weight. Any boffins out there?